Pacemaker manufacturers have been striving to develop satisfactory dual-chambered systems to stimulate the atria and then the ventricles in a sequence that mimics the natural cardiac cycle more closely than do single-chamber systems. Although it has long been known that dual-chamber pacing will give improved physiological cardiac performance and reduced patient morbidity, there have been major obstacles preventing its general use. In particular, there have been problems associated with the atrial leads and the need to have electrode leads in both the right atrium and right ventricle simultaneously.
Until recently, the state-of-the-art in atrial leads was typified by tined Silastic J-shaped leads. (Silastic is a trademark for a silicon rubber composition.) These have typically measured 16 F over folded tines and 10 F over the body of the lead and have to be implanted through a large vein. The unit F indicates French gauge and corresponds to three times the diameter of the lead in millimeters. The Silastic J-shaped leads have been prone to dislodgement and to loss of signal sensing. Their large size and the high friction coefficient of the Silastic coating made the introduction of both ventricular and atrial leads through the same vein cumbersome and difficult.
Recently, several new polyurethane insulated ventricular and atrial leads have been introduced. These leads are thinner and more slippery than their Silastic counterparts because of the use of polyurethane. These properties of polyurethane-insulated leads make it practical to implant two leads through the same vein for dual-chamber pacing. Typically, polyurethane unipolar ventricular leads measure 41/2 F over the body and 9 F over the folded tines. One typical polyurethane atrial J lead measures 8 F over the body and 9 F over the folded tines.
The instant invention relates with particularity to an improvement over existing J-shaped atrial lead designs, arising from certain discoveries concerning atrial anatomy. It was discovered that several important features of the right atrial appendage have been incorrectly or inadequately reported in contemporary medical texts. It was after obtaining a clearer understanding of the structure of the right atrial appendage that the construction of the J-shaped atrial electrode of the instant invention was conceived.
A part of this greater understanding, as illustrated in FIG. 1a, is that the crista terminals originates at the junction of the superior vena cava (SVC) 30 and the right atrial appendage (RAA) 33, as a ridge 35 approximately 10-15 mm deep across the superior end of the entrance of the right atrial appendage. Moreover, the junction of the interior of the tent-like superior margin 31 of the right atrial appendage 33 and the intersection of the superior vena cava 30 and the crista terminalis 35 forms a pocket 36 about 10-15 mm deep. However, contemporary medical texts usually depict the superior margin 31 of the right atrial appendage 33 as springing from a point where the superior vena cava 30 opens into the atrium and neglect the ridge and pocket present at the crista terminalis origin. It is this pocket 36 that is a most suitable location for the placing of the electrode tip 15, of a properly constructed J-shaped atrial lead 11, as shown in FIGS. 1 and 2.
The thin walls of the atrium and most of the right atrial appendage 33 are smooth or have only shallow musculae pectinatae 44 (FIG. 1b) closely attached to the walls. This tissue is not easily gripped by tines attached to a J-shaped atrial electrode. However, the musculae pectinatae of the superior portion of the right atrial appendage are partly detached from the walls and form a limited, localized network to which appropriately designed tines can be anchored. This network of musculae pectinatae is deepest and most prolific in region 37 adjacent to the above-mentioned pocket 36, and becomes shallower at region 38 toward the apex 32 of the right atrial appendage 33.
Commonly, it has been attempted to attach J-shaped atrial electrode leads to this apex 32 of the right atrial appendage 33 as shown in phantom in FIG. 1a. These attempts have not proved entirely successful because the shape and size of the right atrial appendage 33 varies considerably in different patients, ranging from a long, sharply peaked triangle to a short, rounded lobe. Such variations suggest that the placements of previously known J-shaped atrial electrode leads have been imprecise and, therefore, not fully reliable. Even when the external shape of the right atrial appendage has a deeply-pointed peak, and thus appears ideal for locating the tip of a conventional J-shaped atrial electrode, it often occurs that internally, the pointed apex is partly sealed-off either naturally or by cardiac surgery, and not open to the admission of the tip of a J-shaped atrial electrode. Also, the apex 32 of the right atrial appendage 33 is relatively unconstrained and executes large and rapid movements during the cardiac cycle, thus making considerable demands on the ability of conventional J-shaped leads placed in the apex to withstand flexural strain and resist accidental dislodgement. In contrast, the pocket 36 formed at the junction of the superior vena cava 30 and the crista terminalis 35 is comparatively consistent in size and location and undergoes relatively small movements during the cardiac cycle.
Conventional atrial electrode leads 40 (FIG. 1a) have been J-shaped as illustrated by U.S. Pat. No. 3,729,008 issued to Dr. Berkovits on Apr. 24, 1973. As stated above, the J-shape of the lead was selected to hold the electrode tip in contact with the apex 32 of the right atrial appendage 33 or against the superior margin of the right atrial appendage 33 close to the apex 32 and far from the superior vena cava 30. To achieve this, the curvilinear section, i.e., the bend in the J-shaped, of the atrial lead was designed to have a natural radius of curvature of approximately one inch or 25 millimeters.
The J-shaped section was fabricated to be relatively stiff and springy when compared to the general body of the atrial electrode lead. However, it could be bent straight for insertion with a stylet but would spring back to its pre-set J-shape when released into the relaxed position.
Oftentimes, the resilient J-shape has been achieved by fitting a J-shaped wire helix, i.e., the conductor of the pacemaker lead, into a relatively thick and stiff Silastic J-shaped molding. This J-shaped molding was connected to the distal end of a Silastic insulating tube covering the general body of the lead.
Polyurethane J-shaped atrial leads commonly have an extra stiffening helix built into the J-shaped portion to provide the necessary spring effect. This is required because polyurethane takes a set when bent and does not perform well as a spring.